This section enumerates the changes that have been made to Scheme since the ``Revised4 report'' [6] was published.
The report is now a superset of the IEEE standard for Scheme [13]: implementations that conform to the report will also conform to the standard. This required the following changes:
The empty list is now required to count as true.
The classification of features as essential or inessential has been removed. There are now three classes of built-in procedures: primitive, library, and optional. The optional procedures are load, with-input-from-file, with-output-to-file, transcript-on, transcript-off, and interaction-environment, and - and / with more than two arguments. None of these are in the IEEE standard.
Programs are allowed to redefine built-in procedures. Doing so will not change the behavior of other built-in procedures.
Port has been added to the list of disjoint types.
The macro appendix has been removed. High-level macros are now part of the main body of the report. The rewrite rules for derived expressions have been replaced with macro definitions. There are no reserved identifiers.
Syntax-rules now allows vector patterns.
Multiple-value returns, eval, and dynamic-wind have been added.
The calls that are required to be implemented in a properly tail-recursive fashion are defined explicitly.
`@' can be used within identifiers. `|' is reserved
for possible future extensions.